Problem Viewing iTunes Rental Movie

When I view a rental movie on my iMac screen in Full Screen View, movie is small, centered on the screen, about 4" square. Remainder of the screen is black. The same happens for any iMovies I've created.
This problem just started happening. Not sure what changed.
I've downloaded all software updates.
Any ideas??
Thanks

While you're watching it, not in full screen mode - go to View - Video Size - notice the keyboard shortcuts there.

These shortcut keys also work while in Full Screen mode.

Go ahead and switch to FS mode now - hit Command (or Apple) 3, should get you back to full size pic again.

Thank you for the response. I've tried your suggestion and it still does not work.
Another data point: When I watch iMovie via quicktime, I do not have the problem. I"ve uploaded some of the iMovies to iTunes and when I watch them through the iTunes application, I am unable to see them in full screen.